In certain industries there are processes that must be used to bring objects to an extraordinarily high level of cleanliness. For example, in the fabrication of semiconductor wafers, multiple cleaning steps are typically required to remove impurities from the surfaces of the wafers before subsequent processing. The cleaning of a wafer, known as surface preparation, has for years been performed by collecting multiple wafers into a batch and subjecting the batch to a sequence of chemical and rinse steps and eventually to a final drying step. A typical surface preparation procedure involves bathing the wafers in an etch solution of HF and HCI to remove surface oxidation and metallic impurities. Afterwards, the wafers are thoroughly rinsed in high purity deionized water (DI) to remove etch chemicals from the wafers. The rinsed wafers are then dried using one of several known drying processes.
Currently, there are several types of tools and methods used in industry to carry out the surface preparation process. The tool most prevalent in conventional cleaning applications is the immersion wet cleaning platform, or "wet bench." In wet bench processing, a batch of wafers is dipped into a series of process vessels, where certain vessels contain chemicals needed for clean or etch functions, while others contain deionized water ("DI") for the rinsing of these chemicals from the wafer surface. Megasonic energy may be imparted to the wafers using piezoelectric transducers coupled to one or more of the vessels in order to more thoroughly clean the wafer surfaces. In the final process vessel, the rinse fluid is removed from the wafer surface using a solvent such as isopropyl alcohol (IPA). IPA is an organic solvent known to reduce the surface tension of water.
In one IPA drying method, described in U.S. Pat. No. 5,226,242 (Schwenkler), wet substrates are moved into a sealed vessel and placed in the processing region of the vessel. An IPA vapor cloud is generated in a vapor-generating region of the vessel and is directed into the processing region, where it removes water from the wafers. This drying technology is highly effective in removing liquid from the wafers, but is not easily adaptable to single vessel systems in which chemical processing, rinsing, and drying can be carried out in a single vessel.
Environmental concerns have given rise to efforts to improve drying technology in a manner that minimizes IPA usage. One such improved drying technology is the Marongoni technique, which is illustrated schematically in FIG. 1. In one application of the Marongoni technique, an IPA vapor is condensed on top of the rinse water containing the wafers while the wafers are slowly lifted from the processing vessel. The concentration of the dissolved vapor is highest at the wafer surfaces S and lower at regions of the rinse fluid that are spaced from the wafer surfaces. Because surface tension decreases as IPA concentration increases, the surface tension of the water is lowest at the wafer surface where the IPA concentration is highest. The concentration gradient thus results in "Marongoni flow" of the rinse water away from the surfaces of the wafers as indicated by arrow A. Rinse water is thereby stripped from the wafer surfaces, leaving the wafer surfaces dry.
Another application of the Marongoni technique is described in U.S. Pat. No. 4,911,761 (McConnell), which describes a single chamber system for cleaning, rinsing and drying wafers. As described in the patent, a batch of wafers is placed into a single closed vessel, and process fluids are passed from top to bottom sequentially through the vessel. The method further employs a process called "direct displacement drying" to dry the wafers after the final rinse. The drying step is accomplished using an IPA drying vapor introduced into the vessel as the rinse fluid is slowly drained. The IPA vapor displaces the receding rinse water and condenses on the surface of the rinse water in the vessel, creating Marongoni flow from the wafer surfaces into the receding rinse water and resulting in dry wafers.
While providing satisfactory drying results and reducing IPA usage, the direct displacement drying method leaves further room for improvement. For example, because this process relies in part on the pulling (or surface tension) by the descending rinse fluid in the process vessel, it is not adaptable to systems in which rinsing is carried out in a separate vessel and then transferred into a drying vessel. Moreover, the rate at which the deionized water is drained from the vessel must be closely controlled to achieve full benefit of the Marongoni effect.
In a cleaning and drying process described in U.S. Pat. No. 5,571,337 (Mohindra), wafers within a vessel are exposed to process chemicals and subsequently rinsed in DI water to remove residual chemicals. After rinsing, an IPA cleaning step is carried out which utilizes Marongoni flow to remove remaining particles from the wafer surface. This cleaning step involves directing an IPA vapor into the vessel while the DI rinse water is slowly drained, creating Marongoni flow from the wafer surfaces into the receding rinse water. According to the patent, if the rate at which the rinse water recedes is carefully controlled, this flow can be made to carry residual particles away from the wafer surfaces and results in cleaner wafers. In addition to cleaning particles from the wafers, the Marongoni flow during the IPA step removes a substantial amount of rinse water from the wafers. However, water droplets remain on the wafer surfaces at the end of the IPA step, and so hot nitrogen gas is directed onto the wafers to evaporate the residual water droplets. While this process is desirable in that it reduces IPA usage over conventional drying processes, the residual water droplets are problematic in that they may leave impurities on the wafer surfaces.
An object of the present invention is thus to provide an improved drying method and apparatus which is thorough, which minimizes solvent usage, and which is highly adaptable for use in a variety of surface preparation systems and processes.